Shadow mask type color picture tubes usually include a screen of red, green and blue-emitting phosphor lines or dots, an electron gun for exciting the screen, and a shadow mask interposed between the gun and the screen. The shadow mask is a thin multiapertured sheet of metal precisely disposed adjacent the screen so that the mask apertures are systematically related to the phosphor lines or dots.
Color picture tubes having shadow masks with slit shaped apertures have received relatively recent commercial acceptance. One of the reasons for this acceptance is that the electron beam transmission through the mask can be made higher for a slit-mask, line-screen type of tube than for a circular-apertured mask, dot-screen type tube. Even though the use of a slit mask provides a definite advantage in electron beam transmission, however, this transmission can be increased even further than is practiced in the present art.
In one type of slit shadow mask, the mask has vertically extending slit apertures which are interrupted by a plurality of spaced bridges or webs which provide mechanical rigidity. The presence of these webs, however, has an effect on electron beam transmission and thus on luminescent brightness. Such effect is greatest at the four corners of the mask, because the angles between the electron beams and a normal to the mask are greatest at the corners. With these greater angles, the electron beams strike not only the surfaces of the webs but also portions of the webs that form the ends of the slit apertures. Therefore, it is desirable to develop a mask fabrication technique which will produce a shadow mask wherein the parts of the electron beams striking the portions of the webs that form the ends of the slit apertures will be reduced or eliminated.
Shadow masks are fabricated from rolls of metal sheets, utilizing a photographic method. Such fabrication is disclosed in e.g., U.S. Pat. No. 2,750,524, issued to F. G. Braham on June 12, 1956; U.S. Pat. No. 3,199,430, issued to S. A. Brown on Aug. 10, 1965; U.S. Pat. No. 3,313,225, issued to N. B. Mears on Apr. 11, 1967; and U.S. Pat. No. 3,751,250, issued to J. J. Moscony et al. on Aug. 7, 1973. These patents are hereby incorporated by reference for the purpose of their disclosures of mask fabrication methods and related equipment.
The first step in the fabrication is to coat both sides of the metal sheet with a photosensitive material. Thereafter, the sides are photoexposed through two aligned photographic masters. Each photographic master has an array of elements corresponding to the apertures desired in the shadow mask. The elements of one master are larger than the elements of the other master so that the resultant mask apertures have larger openings facing the tube screen than facing the electron gun. Following exposure, the photosensitive material on the metal sheet is developed, and the photosensitive material at the areas corresponding to aperture locations is removed, thus exposing the metal sheet at these locations. Next, the metal sheet is etched to form apertures therethrough at the exposed locations. To obtain a desired aperture shape and a corresponding web shape, it is necessary to appropriately shape the pattern elements and position the corresponding elements of the two photographic masters.